Terms that are used below in the presentations of the prior art and of the invention are explained first with reference to FIGS. 5a and b. FIG. 5a schematically depicts a partial plan view of the luminous screen of a color picture tube, while FIG. 5b illustrates the density distribution of an electron beam.
The luminous screen according to FIG. 5a has matrix stripes 10 and phosphor stripes 11 for three different colors, indicated by R for red, G for green, and B for blue. Associated with each color is an electron beam that is scanned in the x and y directions over the luminous screen. Electrons then penetrate, in the form of a spot 12, through a shadow mask (not depicted) and strike the luminous screen. In the example according to FIG. 5a, each spot has a height of 700 .mu.m and a width of 220 .mu.m. The vertical spacing between the beginnings of adjacent spots is 800 .mu.m, while the horizontal spacing is 260 .mu.m. However, the spots do not produce a luminous effect over their entire area, but only in that region in which a spot strikes a phosphor stripe. It should be mentioned that the aforesaid dimensions apply essentially to the center of the color picture tube. Towards the edge in the horizontal direction, the widths of the phosphor stripes get larger and the spot widths smaller. In the vertical direction, the spot widths decrease toward the edge.
While a luminous region, as just defined, is that area of the luminous screen in which an individual electron spot produces a luminous effect, a luminous segment is a portion of a luminous region that is perceptible through the hole 13 of a mask. The term "luminous domains" is used as a general term. Luminous domains can be any luminous areas, for example only portions of a segment, or a plurality of luminous segments or regions domains can be any luminous areas, for example only portions of a segment, or a plurality of luminous segments or regions together.
Drawn in FIG. 5a as a dashed segment of a circle is a line that is designed to illustrate the boundary of a stationary electron beam. This boundary is drawn at 5% of the maximum brightness of the beam. The brightness distribution is depicted in FIG. 5b. It corresponds essentially to the electron density distribution; the latter can therefore be determined by measuring the brightness distribution produced by the electron beam on the luminescent screen.
If no shadow mask were present, if the screen were homogeneously coated with phosphor, and if the electron beam were well formed, a circular electron spot would strike the luminescent screen and generate on it a circular luminous spot whose periphery would coincide with the dashed line 14 of FIG. 5a. However, only a few regions of this hypothetical luminous spot actually luminesce; these are indicated in FIG. 5a by crosshatching in the phosphor stripes G. It should be mentioned at this point that when a color picture tube is operated with a single electron beam, not all the spots 12 according to FIG. 5a are perceptible, but only those that belong to the electron beam that was just emitted, for example the one that is designed to excite luminescence in the green-emitting phosphor stripes Only those regions of these phosphor stripes that lie within the circle with the periphery 14 will in turn luminesce. In what follows, when an electron spot is discussed, it is to be understood as the electron spot that would be generated by a single electron beam if the shadow mask were absent A "luminous spot" is understood to mean the area that such a hypothetical electron spot would generate if the luminescent screen were homogeneously coated with phosphor.
In a known process for measuring electron density distribution, viewed over the cross section of an electron beam in a color picture tube, a luminous spot is observed through the mask hole 13 of FIG. 5a with a light-sensitive diode which, in some circumstances with an additional optical system, acts as a camera. The mask shields the diode from all light except that which penetrates through the mask hole 13. The electron beam is then aligned so that its center coincides with the mask hole 13 (beginning at the position in FIG. 5a, it would therefore need to be displaced slightly upward) Then the electron beam is moved horizontally and vertically under the mask hole. This produces a profile of brightness over time, as drawn in FIG. 5b as a local profile for brightness along the horizontal center line H (FIG. 5a). The brightness distribution along the vertical center line looks the same when the electron beam is circularly symmetrical.
The known process therefore possesses the following steps:
displace the electron beam; PA0 record the images of luminous spot domains that are perceptible, as the electron beam is displaced, due to the effect of a masking means; and PA0 analyze the recorded images to obtain a result concerning the luminance distribution produced by the electron beam, which is essentially identical to the electron density distribution. PA0 a masking means; PA0 a camera (19) to record the images of luminous spot domains that are perceptible due to the action of the masking means; PA0 a driving arrangement (18) to drive a deflection arrangement (17) on the color picture tube in such a way that the electron beam is displaced with respect to the camera; and PA0 an analysis arrangement (20) to analyze the images recorded by the camera, in order to obtain a result concerning the luminance distribution produced by the electron beam, which is essentially identical to the electron density distribution. PA0 the camera is an image converter camera (19) and PA0 a sequence controller (21) is present, which is designed so that PA0 to define the electron density distribution, PA0 to record the images, an image converter camera is used; PA0 to generate a simulated masking means, luminous spots are generated in the entire field of view of the image converter, and the image converter regions selected for later use are those that lie within those image converter regions in which the luminous spots are imaged, with these selected image converter regions acting as simulated masking slots of a simulated masking means through which the luminous spot segments are perceptible; and PA0 to define the electron density distribution,
The known device possesses the following features
With this process and the associated device, the mask must be aligned with the center of a phosphor stripe, and the electron beam must be moved in the x and y directions behind the mask hole so that its center coincides in each case with the center of the mask hole. These laborious alignment procedures must be repeated for each of three electron beams at different screen locations in order to examine the beam shape for an entire screen.
As is evident from the above, the problem that existed was to indicate a process and a device of the aforesaid type that requires no alignment effort.